The invention relates to a digital solid-state trip unit for an electrical circuit breaker. The trip units of the kind mentioned comprise long delay and possibly short delay tripping functions, selected to provide protection to an item of equipment, for example a motor which overheats when it is supplied by an overload current. These functions are of the inverse time type represented by the relation I.sup.2 t=constant, in which I is the current and t the time. The simple calculation of this function does not take the former state into account, notably the overheating of the protected equipment and does not provide reliable protection.
The object of the present invention is to enable an inverse time function to be performed by simple digital processing.